1. Field of the Invention
This invention relates generally to fabrication of integrated circuit packages, and in particular, to fabrication of a plurality of integrated circuit packages having coplanar heat sinks attached thereto.
2. Description of the Related Technology
Integrated circuits have revolutionized the field of electronics by making possible a level of technological sophistication unknown in the days of vacuum tubes and even discrete transistors. An integrated circuit die may comprise, on a small silicon chip, many thousands or even a million or more transistors interconnected together to form complex electronic functions. The complex electronic functions of the integrated circuit chip my require several hundred external connections to a related electronic system.
Simple function integrated circuits have been packaged in ceramic packages for high reliability industrial and military applications, and in lower cost molded plastic packages for commercial and consumer products. Recently, very large scale integration (VLSI) integrated circuits have outgrown the connection capacity of either the ceramic or molded plastic packaging systems. The integrated circuit packaging industry has developed more sophisticated packages for VLSI integrated circuits that accommodate the increased number of external connections required to the electronic system.
Several of the VLSI integrated circuit packages having high connection capacity are a plastic pin grid array (PPGA) and a plastic ball grid array (PBGA). The PPGA and PBGA packages differ from the prior molded plastic or ceramic packages in that the PPGA and PBGA are, in effect, miniature multiple layer printed circuit boards having the integrated circuit chip contained within the multiple layers and connected to the various conductive paths of the printed circuit boards.
Interconnections are made between conductors of the different board layers by plated through holes or "vias". The thickness of the boards laminated together limits the size of the interconnection vias. The thicker the laminated boards or overall lamination thickness, the larger must be the diameter of the vias. In complex integrated circuit packages, many hundred interconnections are required. Thus, it is preferable to make the via diameter as small as practical so that interconnecting conductive traces may be routed within the package.
The various layers of printed circuit or wiring boards may be laminated together using adhesive or bonding sheets made up of "B-stage" or "prepreg" material placed between the wiring boards. The wiring board may be comprised of C-stage material. C-stage material is a high temperature glass fiber reinforced epoxy laminate. B-stage is an intermediate stage in the reaction of thermosetting resin in which the material softens when subject to pressure and heat but does not entirely fuse. The B-stage material typically is of the same type of material as is the C-stage material, but only in a partially cured state.
The PPGA and PBGA differ mainly in that the PPGA utilizes conductive metal pins that may be either soldered to a system printed circuit board or inserted into a matching socket which is already soldered to the system printed circuit board. The PBGA utilizes solder "balls" instead of metal pins. The PBGA solder balls reflow to connection points on a printed circuit board when heated to a certain temperature, thus electrically connecting the circuitry within the PBGA integrated circuit package to the electronic system.
The PPGA and PBGA packages, including adaptations thereof for surface mount and hybrid applications, utilize a plurality of multi-layer printed wiring board (PWB) planar substrates made up, for example, of polyimide, glass reinforced epoxy, or other materials known to those skilled in the art of fabricating very large scale integrated circuit packages. Some of the planar substrates have material cut out from the middle, and when laminated together, form a cavity in which the integrated circuit die may be placed.
After the integrated circuit die is installed in the package, connections are made to the die connection pads and the conductive metal patterns of one or more of the planar substrates. The planar substrate metal patterns also connect to either the connection pins or connection solder balls of the PPGA or PBGA package, respectively. Thus, the PPGA or PBGA package is a miniature multi-layer printed circuit board system containing the integrated circuit die and forming a housing for and protection of the die. The die is further protected by encapsulating it with plastic or epoxy material. Examples of integrated circuit fabrication for VLSI integrated circuit packages are more fully illustrated in co-pending patent application Ser. No. 07/917,894 entitled "Ball Bump Grid Array Semiconductor Packages" by Michael Rostoker, Chok J. Chia, Mark Schneider, Michael Steidl, Edwin Fulcher and Keith Newman, filed on Jul. 21, 1992 and incorporated by reference herein for all purposes.
The complex circuitry of sophisticated integrated circuit chip dice generate heat which must be dissipated or else thermal destruction may result. Encapsulation of the chip die, wherein additional plastic material is molded around the chip, further exasperates efficient heat dissipation. A heat spreader or heat sink made of thermally conductive material such as, for example, copper, copper tungsten, ceramics, thermally conductive filled polymers, or aluminum has been incorporated with the integrated circuit package.
The heat spreader is normally attached in close proximity with a major face of the integrated circuit die and is in thermal communication therewith. This heat spreader is adapted to conduct heat from the circuitry of the die by thermal convection and/or conduction. The heat spreader may be a lead frame used to connect the integrated circuit die to external circuits, or may be a separate metal plate that is part of the integrated circuit package. In the higher power dissipation integrated circuits, a metal plate, attached to a major face of the die, is utilized. An additional heat spreader/heat sink may be attached to the metal plate to increase the heat dissipating surface area for more efficient heat transfer.
Accordingly, it is desirable to provide a method and system for simply and cost efficiently manufacturing large scale integrated circuit packages having integral heat spreaders on which integrated circuit dice are attached thereto.